(1) Field of the Invention
This invention relates to an apparatus and a method for molding hollow thin-walled articles from a gellable material such as vinyl plastisol.
Various vinyl plastisol compositions are known which can be used in various powdered vinyl molding processes. Typically, an open hollow mold is covered by dry plastisol and heat is applied to the mold surface and transferred to the dry plastisol to gel a layer of plastisol adjacent the mold surface. The thickness of the gelled layer depends upon the temperature to which the plastisol layer is subjected and the time the plastisol is maintained at that temperature. After the layer adjacent the mold surface is gelled, the ungelled plastisol is poured out of the mold. In such processes, certain mold configurations may have surfaces thereon in which deposited material is later disposed of as scrap or trim waste.
(2) Description of the Prior Art
Articles which are manufactured by vinyl cast on a heated mold often include salvage or trim areas which are not used in the final product and therefore do not require the thickness of the other finished areas in the product. Yet, when the entire mold surface is heated uniformly the entire finished article has approximately the same thickness even though some areas are trim or non-usable. There is known in the slush molding art the technique of varying the thickness by heating different areas of the mold to a higher degree to increase the thickness of the article in certain areas. Such a technique is exemplified in U.S. Pat. No. 2,588,571 granted to Sydney Porter on Mar. 11, 1952. That patent discloses the technique of slush molding a boot by utilization of infrared lamp heating and shielding the heating from certain portions of the mold to prevent the build-up of the plastisol thickness and to increase the thickness of the sole of the boot by increased heating.
Another apparatus for controlling waste build-up on a mold is formed in a mold having first and second groups of liquid passages disposed adjacent the mold surface for controlling the temperature over respective first and second areas of the mold surface as set forth in U.S. Pat. No. 4,217,325 granted to D. E. Colby on Aug. 12, 1980 with a common assignee. The first group of liquid passages are disposed adjacent the first area of the mold surface where it is desired to increase the thickness of the finished article whereas the second group of passages is disposed adjacent the second area of the mold surface where the material in the finished article is scrapped and/or will be trimmed and therefore the desired thickness is desirably less. Both the first and second groups of passages are supplied with liquid to maintain the temperature of the first and second areas of the mold surface at a non-gelling temperature as liquid plastisol is disposed over the mold surface to define a coating or layer. This very thin coating of plastisol is applied to be free of surface blemishes. Thereafter, additional liquid plastisol is provided for increasing the thickness only over the first area of the mold surface. To accomplish this, the first group of passages is supplied with a liquid at a temperature higher than the non-gelling temperature of the liquid in the second group of passages for heating the first area of the mold surface to a higher temperature than the second area so as to gel the liquid plastisol over the first area of the mold surface which is heated with the heat supplied by the liquid in the first group of passages. After the desired gelled thickness is attained, all passages are supplied with liquid at a temperature sufficient to heat the first and second areas of the mold to a cure temperature for curing all of the plastisol. Accordingly, the finished article will have a greater thickness over the first area which defines the finished area of the article whereas the thickness of the article over the second area of the mold surface will be much thinner and define the scrap or trim areas. Consequently, significant amounts of material are saved.
Still another method of controlling the build-up of gelled material on a mold surface is set forth in U.S. Ser. No. 673,810 filed Nov. 21, 1984 for Gas Conditioned Modular Slush Molding Machine. In the '810 case, the control of waste build-up is accomplished by selective control of air flow from a heated plenum through elongated tubes with nozzles that impinge heated air onto exposed surfaces of the mold.
U.S. Pat. No. 3,698,849 discloses an injection molding nozzle with an insulating gasket that enables the nozzle to remain seated on a sprue bush during the whole of a molding cycle.